1. Field of the Invention
The present invention relates to a network termination enabling relief of call congestion in an exchanger provided in a digital communication network, such as an ISDN (Integrated Services Digital Network).
2. Description of the Related Art
Recently, the ISDN is widely utilized as a digital communication network to achieve high speed digital data transmission, such as telephone communication, facsimile communication, computer data communication and the like. Further, recently, a B-ISDN is developed for high speed digital communication. In the B-ISDN, it is possible to perform various digital communication, each having different transmission speed for example, CATV, telephone, GIII facsimile, and the like on the same network. A SDH (Synchronous Digital Hierarchy) method is employed in the B-ISDN, and an ATM (Asynchronous Transfer Mode) method is also employed in combination with the SDH method to achieve high speed digital data transmission. In general, a plurality of exchangers are provided in an ISDN, and a plurality of terminal adaptors are connected to each of exchangers through a network termination. The terminal adaptors are provided for users. Further, the SDH method utilizes a plurality of ATM cells to transfer the data from the exchanger to the terminal adaptors, and from the terminal adaptor to the exchanger. As explained in detail below, a SDH frame has a plurality of ATM cells, and one ATM cell is constituted by a header field and an information field.
Sometimes, the exchanger becomes congested with calls caused by an overflow of calls from the end terminal adaptor. In this case, "call" means the state where the data fills the information field of the ATM cell. Accordingly, it is necessary to limit the transfer of the data from the terminal adaptor to the exchanger to relieve the call congestion in the exchanger.